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Abstract

In this study, the sound properties of four types of nanocomposites have been investigated. To this end, the prepared samples were measured by the impedance tube model BSWA-SW 422, SW477. It was found that the Sound Absorption Coefficient (SAC) of all samples was increased at high frequencies relatively well. The highest SAC at medium and low frequencies was related to the nanocomposite D. The results of sound Transmission Loss (TL) of nanocomposites showed that the TL value for the nanocomposite D (optimum sample) was higher at all frequencies compared to other nanocomposites. The results confirmed that adding organic and mineral materials to the silica aerogel (SA) simultaneously improves its sound properties. By measuring the Sound Pressure Level (SPL) around the enclosure without optimum sample and with optimum sample in the four sound ranges, we found that using nanocomposite D can significantly reduce the noise. According to this study, SA/polyester nonwoven layer/pan nanofibers/nanoclay nanocomposite (nanocomposites D) have great sound absorption properties, which can be used in different environments.

Keywords

Sound properties silica aerogel nanocomposites noise pollution

Introduction

Noise pollution is one of the most important environmental pollutants that endanger the human health in various aspects. Noise can have adverse effects on human health such as hearing loss, hypertension and increased physiological and psychological stress.^1-3 Sound insulation and sound absorbers can be used to control the noise in the environment. Sometimes, sound insulating materials are confused with sound absorbing materials. Sound insulating materials are non-porous, high-density materials that can prevent the noise passage from one place to another. On the other hand, sound absorbing materials are low-density, porous materials that can prevent sound the reflection.⁴

Porous adsorbent materials can significantly help to absorb the sound. These materials have open cavities or pores containing air. When sound waves enter through these pores, due to the friction between the movement of the air particles and the cavities within the material, sound kinetic energy is converted to heat energy and the sound is attenuated.⁵ The aerogels were discovered by Kistler in the 1930s which can be used for this purpose. He first produced silica aerogel with the method of replacing the liquid phase of the gel by gas with only an inconsiderable shrinkage of the gel.⁶

Aerogels are highly porous materials with extremely low density (<0.05 g/cm³) and high specific surface area (>1000 g/m²). Their nanoporous structures are composed of pores with diameters in the range of 2-50 nm (i.e. mesopores).^7,8 It has high porosity (90-99%) and very low thermal conductivity of 0.013 W/m/K which can be exploited in designing effective materials for sound absorption.^9-17 Moreover, the speed of sound in aerogels is lower than air and they have exceptional sound properties. The sound properties of silica aerogel have been extensively studied by Gibiat et al.^10,11 Lightweight materials are generally not suitable for sound insulation. However, low-density lightweight aerogels are suitable for sound insulation, which has been studied by various researchers.^4,12,13 Silica aerogel is the most common type of aerogel; a porous material with nanometer-sized cavities and excellent sound capabilities. Silica aerogel is unique in its properties, however, as silica aerogel has high porosity it is crisp and has low mechanical properties.

In this regard, Chen and Jiang¹⁴ investigated the sound properties of polyurethane foam composites with bamboo chips and stems, for which, the results showed that the composite bamboo chips and stems cause a significant increase in the sound absorption and insulation at all frequencies, especially at low frequencies.¹⁴ Eskandari et al.¹¹ studied the thermal and sound properties of silica aerogel (SA)/UPVC composites. The results displayed that the addition of SA to the UPVC matrix increased the sound absorption property due to the high porosity up to three times and increased the transmission loss in the low frequency range.¹¹ Rabbi et al.¹⁵ by examining the addition of nanofibers layers to nonwoven layers, found a significant increase in the Sound Absorption Coefficient (SAC). In particular, the SAC of polyester nonwoven layers was increased from 0.40 to 0.67 and 0.71 with the presence of polyurethane (PU) and polyacrylonitrile (PAN) nanofibers layers at a frequency of 2000 Hz.¹⁵ Orfali et al.¹⁶ reported that by adding 0.2 wt% silica nano-powder and 0.35 wt% carbon nanotube to the polyurethane compound, sound Transmission Loss (TL) can be improved by up to 80 dB compared to the pure polyurethane foam samples.¹⁶

Buratti et al.¹⁷ studied the sound properties of natural gypsum composite aerogel granules with different percentages, the results of which indicated that the aerogel-based gypsum increased the sound absorption. Feng et al.³ prepared cellulose silica aerosols from recycled cellulose fibers and methyltrimethoxysilane (MTMS). Cellulose silica aerogels provided a good thermal and sound insulation properties compared to silica aerogels and cellulose aerogels.³ Venkataraman et al. have studied the sound properties of aerogels combined with nonwoven textiles and reported that the aerogels combined with nonwoven textiles have higher SAC than aerogels and are suitable for acoustic applications.¹⁸ Dong et al.¹⁹ investigated the sound properties of organic and inorganic aerogel composites in which the SA composite (with different concentrations of silica and polydimethylsiloxane) had adsorption properties better than glass wool.¹⁹ Ahmadi et al. by examining the sound properties of the 3D printed earmuff nanocomposite, observed that the use of clay/acrylonitrile butadiene styrene (ABS) nanocomposites in earmuff increases the noise reduction power.²⁰ Dourbash et al.²¹ indicated that the addition of SA to the sponge polyurethane did not have a good effect on its sound properties, however, adding SA to the elastomeric polyurethane improved the sound insulation properties. Moradi et al. studied the PMMA composition in polyurethane cross-linked polymer networks and showed that due to the permeability of the two polymers, its sound damping properties was increased.²²

The present work was aimed at synthesizing novel nanocomposites which can be efficiently engineered to induce great sound properties; to this end, organic (nonwoven polyester layer and pan nanofibers) and mineral (nanoclay particles) materials have been added to the silica aerogel. The sound properties of nanocomposite containing SA have not been adequately investigated elsewhere and herein the sound absorption and sound insulation capability of the nanocomposites containing SA and sound pressure reduction rate of the optimum nanocomposite have been investigated.

Experimental

Materials

Silicon dioxide (SiO₂), absolute ethanol, tetraethyl orthosilicate (TEOS), ethylmethyl ketone, ammonium hydroxide (NH₄OH), hexamethyldisiloxane were purchased from Merck (Germany) and were used to prepare silica aerogel. The polyacrylonitrile (PAN) powder (Mw 10⁵ g/mol) was purchased from Iran Polyacryl Company. Dimethylformamide (DMF) was obtained from Merck (Germany) as a solvent with a purity of more than 99% and density of 0.948 g/m³. Nano clay montmorillonite K (10) was supplied from Sigma-Aldrich Company with a density of 0.5-0.7 g/cm³, 2% moisture, and particle size of 1-2 nm. Polyester nonwoven layers were supplied from Parsian Aliaf Media Company (Iran) with an average surface unit weight of 150 g/m ², and were used as a bedding material for preparation of nanocomposites.

Preparation of nanocomposites

In this research, 20 wt% of TEOS and ethanol were added to SiO₂. It was diluted with ethanol with ethyl methyl ketone as a precursor of sol silica. A 5.5 M solution of ammonium hydroxide was added to 2 vol% silica sol. Activated silica sol was poured into a mold in which the samples were combined or separated. Hexamethyldisiloxane was used for surface modification. It was placed in the oven at 150°C for 3 hours.²³ In this research, four types of nanocomposite have prepared including:

Type A: Polyester nonwoven layers/SA nanocomposite

Type B: Polyester nonwoven layers, nanofiber pan/SA nanocomposite

Type C: Polyester nonwoven layers, nanoclay/SA nanocomposite

Type D: Polyester nonwoven layers, nanofiber pan, nanoclay/SA nanocomposite

The nanocomposites were prepared with a thickness of 2 cm. In order to determine the SAC and TL of the nanocomposites, they were prepared with 100 and 30 mm diameter.

Measurement of SAC

Sound properties including SAC and TL were measured according to the international standards using an impedance tube (model BSWA-SW422, SW477). The sound properties of samples by the impedance tube was measured in a frequency range of 80-6300 Hz according to standard ISO 10534-2. The SAC was determined using two microphones as well as TL by using four microphones by impedance tube. Diameter of impedance tubes was equal to 100 mm (for frequencies ranged from 80–1.6 kHz) and 30 mm (for frequencies ranged from 1600–6.3 kHz). Moreover, four microphones 1/4″ with ICP preamplifier were used for performing the necessary measurements. VA-Lab IMP impedance tube software was applied based on the LabVIEW platform.

Electrospinning apparatus

To prepare the nanofibers layer, a horizontal-type electrospinning device with a cylindrical collector was used. Highly homogenous polyacrylonitrile (PAN) polymer solution (13 wt%) was obtained by dissolving the polymer in dimethylformamide solvent by magnetic stirrer for 24 hours at 20°C. It was added to a syringe containing needle (needle gauge 20), to create an electric field between needles and collectors from a high voltage source capable of producing DC voltage up to 18 KV. The feed rate of the polymer solution was adjusted by a pump at 45 ml/min. In order to achieve a uniform nanofiber layer on the collector, the pump with a syringe had a reciprocating motion and all samples were produced under the identical conditions.

Measurement of sound pressure level (SPL)

In the first step, a cylindrical sound source with a length of 5 cm and a diameter of 5.5 cm was prepared. Measurement of SPL was performed at eight points around the sound source at a distance of 1 meter in the four sound ranges of 75-80, 80-85, 85-90, 90-95 dB with frequency analysis in the C-weighting network using sound level meter model CASSELA-CEL-62X according to ISO 9612 standard. The sound level meter was calibrated using the calibrator model CEL-110/2.

In the second step, an enclosure with dimensions of 20 × 13 × 9.5 cm³ of wood, with a thickness of 1.3 cm was made. The wood enclosure was considered because wood is one of materials used in various environments for noise reduction. An enclosure with these dimensions was considered because the amount of material used in the synthesis of nanocomposites was predictable for testing in the laboratory environment and for embedding in an enclosure with these dimensions.

A sound source was placed inside the enclosure. The SPL was measured in the four sound ranges of 75-80, 80-85, 85-90, and 90-95 dB at eight points at a distance of 1 meter around the enclosure.

In the third step, to fit the optimum nanocomposite (nanocomposite that has better SPL and TL) in the enclosure, two nanocomposites were prepared with dimensions of 6 × 6 cm² for the ceiling of the enclosure, and eight nanocomposites were prepared with dimensions of 14 × 6 cm² for walls and floor of the enclosure. Nanocomposites were prepared with a thickness of 2 cm and were placed in the enclosure. A sound source was inserted inside the enclosure containing nanocomposites. The SPL measurements were performed in the four sound ranges at eight points at a distance of 1 meter around the enclosure containing the nanocomposites.

Characterization

Morphology of the pristine SA and the synthesized nanocomposite have been studied by using Field Emission Scanning Electron Microscopy (FE-SEM, ZEISS Company, Germany, Model SIGMA VP). Furthermore, the mean pore diameter, total pore volume, specific surface area of samples have been determined through nitrogen adsorption/desorption experiments through BET and BJH methods (BELSORP, Mini of Company Microtrac Bel Corp).

Results and discussion

Characterization

The morphology of the SA particles and the nanocomposite were characterized by Field Emission Scanning Electron Microscopy (FE-SEM). Figure 1 shows the micrograph of the SA nanoparticles. In this figure, the porous network structure of the SA can be detected in which the nanoparticles with the diameter of 20-50 nm have been synthesized. This figure confirms the successful synthesis of SA nanoparticles. Figure 2 shows the FE-SEM image of the nanocomposite type D. This displayed a very good coverage of nanofibers and nanoclay surface by SA particles. The distribution of aerogels particles was uniform on the surface of nanofibers and nanoclay. Based on the results of FE-SEM, the SA particles (Figure 1) were agglomerated and interconnected and had nanometer pores. Cavities and porosities contribute to the sound absorption. The SA nanoparticles diameter was approximately in the range of 20-40 nm. These particles contained 44.4 wt% oxygen and 55.6 wt% Si (determined through Energy-dispersive X-ray spectroscopy). In the optimum nanocomposite (i.e. type D), the SA nanoparticles and nanofibers were fully interconnected and dispersed as shown in Figure 2. The SA and nanoclay particles could be dispersed on the surface of nanofibers and nonwoven layers. The nanocomposite contained 52.1 wt% carbon, 24.1 wt% oxygen, 11.7 wt% nitrogen, and 10.1 wt% silicon.

Figure 1.

The FE-SEM image of the pristine silica aerogel nanoparticles.

Figure 2.

The FE-SEM image of SA/polyester nonwoven layer/nanofibers pan/nanoclay nanocomposite.

In addition, the specific surface area, mean pore diameter and total pore volume of the SA naoparticles and the nanocomposites, based on the results of nitrogen adsorption/desorption experiments, showed that the prepared SA nanoparticles had the specific surface area of 183.54 m²/g, mean pore diameter of 50.73 nm and total pore volume of 2.3278 cm³/g. The nanocomposite type D has also been characterized through N₂ adsorption/desorption isotherms for which a specific surface area of 28.487 m²/g, mean pore diameter of 44.004 nm and total pore volume of 0.3134 cm³/g was achieved. Altogether, it can be stated that a multicomponent nanocomposite has been prepared successfully which can be used to reduce the noise pollution. Based on the results of the nitrogen adsorption/desorption experiments, the specific surface area, mean pore diameter, and total pore volume of the SA nanoparticles were higher than the optimum nanocomposite (i.e. Type D). This is because in the optimum nanocomposite, the SA nanoparticles were interconnected with the nanofibers and nanoclay and the surface of the nanocomposite became denser and more compact and its specific surface area was decreased. The nanofibers and nanoclay particles acted as a Skeleton in this nanocomposite and increased the mechanical properties of silica aerogels. It can be stated that the SA nanoparticles with nanofibers and nanoclay particles were still highly porous, so the nanocomposite containing SA could have great sound absorption properties.

Sound absorption coefficient (SAC) of the nanocomposites

Figure 3 illustrates the SAC results of the prepared nanocomposites (i.e. types A, B, C, and D) at different frequencies (80-6300 Hz). According to Figure 3, the SAC of the samples was increased with increasing the frequency and the SAC of the samples showed a relatively good increase at high frequencies.

Figure 3.

The SAC values of A: SA/polyester nonwoven layer nanocomposite; B: SA/pan nanofibers nanocomposite; C: SA/nanoclay nanocomposite; and D: SA/polyester nonwoven layer/pan nanofibers/nanoclay nanocomposite.

The increase in the SAC at high frequencies for these nanocomposites was in the following order: B, A, D, and C. At high frequencies the highest SAC was related to the nanocomposite B by 0.98 at a frequency of 3150 Hz. The SAC of nanocomposite B was reduced at medium and low frequencies compared to other samples. In general, the SAC was weak and limited at medium and low frequencies. Figure 3 demonstrates that the highest SAC correspond to the nanocomposite D at medium and low frequencies. The SAC of nanocomposite D at frequencies of 400, 500, 1000, 1250, and 2000 Hz were obtained to be 0.38, 0.51, 0.78, 0.83, and 0.84, respectively. Only the nanocomposite type D showed a good sound absorption compared to other nanocomposites at medium and low frequencies.

Sound transmission loss (TL) of the nanocomposites

In Figure 4, the TL values of the nanocomposites are compared at different frequencies (80-6300 Hz). It can be seen from Figure 4 that the TL values of all nanocomposites were in the range of 1.4 to 9.3 dB. The TL values of all nanocomposites reached their peak at a frequency of 6300 HZ.

Figure 4.

The sound TL values of A: SA/polyester nonwoven layer nanocomposite; B: SA/pan nanofibers nanocomposite; C: SA/nanoclay nanocomposite; and D: SA/polyester nonwoven layer/pan nanofibers/nanoclay nanocomposite.

The TL value of nanocomposites A, B, C, D at a frequency of 6300 Hz were 4.35, 4.2, 8, and 9.3 dB, respectively. The TL of the nanocomposite D was higher at all frequencies compared to other ones. The TL of nanocomposite D at high frequencies was higher than that at the low frequencies. The TL values of the nanocomposite D at frequencies of 400, 1000, 2000, 3150, 4000 Hz were 3.4, 5.2, 4.6, 5.5, and 5.7 dB, respectively. The lowest TL value of the nanocomposite D at a frequency of 200 Hz was approximately 2.1 dB. According to the results of the SAC and TL measurements, it can be deduced that the nanocomposite D had the better performance in terms of SAC and TL compared to other samples. Therefore, nanocomposite D was considered as an optimum sample in this research.

An overview of studies on sound properties of various materials is presented in Table 1. The sound properties of the polyester material, polyurethane, PVC, etc. has been reported in the literature and compared in the current work. Measurements showed that addition of nanofibers layers within nonwoven layers leads to a significant increase in SAC. In particular, SAC of polyester nonwoven layers increased from 0.40 to 0.67 and 0.71 with the presence of PU and PAN nanofibers at frequency of 2000 Hz, respectively.¹⁵

Table 1.

A review of studies on the sound properties of various materials.

No.	Materials	Acoustic properties	Ref.
1	SA/UPVC	The sound absorption of SA/UPVC composites were increased up to three times in comparison to the pure UPVC. The maximum TL in this composite was 21% higher than that of the pure UPVC.	¹¹
2	CNT/SiO₂ nano-powder/polyurethane	Addition of 0.2 wt% Silicon Oxide Nano-powder and 0.35 wt% CNT to PU improved the TL by up to 80 dB than the pure PU foam.	¹⁶
3	SA/PU	Unlike foam composites, adding SA to elastomeric PU improved the acoustic insulation properties. The highest TL at frequencies lower than 3000 Hz was observed in a sample containing only 1 wt% SA.	²¹
4	PAN and PU Nanofiber/polyester and wool Nonwoven	The nanofiber layers in nonwoven materials could improve both the SAC and TL simultaneously, especially at medium and lower frequencies.	¹⁵
5	Polyurethane foam/bamboo leaves particles	For the Pu/6% bamboo the SAC was 0.971 at frequency of 6300 Hz. The Pu/8% 2-3 mm bamboo chips had the high TL of 18.9 dB in the frequency range of 100-6300	¹⁴
6	SA/polyester nonwoven layer/pan nanofibers/nanoclay	The SAC values were 0.38, 0.51, 0.78, 0.83, and 0.84 at frequencies of 400, 500, 1000, 1250 and 2000 Hz, respectively. The TL values were 3.4, 5.2, 4.6, 5.5, 5.7 dB at frequencies of 400, 1000, 2000, 3150 and 4000 Hz, respectively.	Herein

By examining the sound properties of the SA /UPVC composites, the results showed that adding SA to the UPVC matrix increased sound absorption property by three times due to high porosity and increased the TL in the low frequency range.¹¹

Addition of SA to sponge polyurethane does not have a favorable effect on its sound properties, but by adding SA to elastomeric polyurethane it improves its sound insulating properties.²¹

By examining the sound properties of polyurethane foam composites produced with bamboo chips and stems, the results showed that bamboo composite chips and stems significantly increase sound absorption and insulation at all frequencies, especially at low frequencies.¹⁴

The addition of 0.2 wt% silicon oxide nano-powder and 0.35 wt% carbon nanotube to the polyurethane blend improved the sound TL (sound absorption) by up to 80 dB compared to pure polyurethane foam sample.¹⁶

Our results showed that adding organic (nonwoven polyester layer and pan nanofibers) and mineral (nanoclay particles) materials simultaneously and together to SA could improve the SAC and TL values.

Sound pressure level (SPL) of the nanocomposites

The results of the average SPL measurements in the sound ranges of 75-80, 80-85, 85-90, and 90-95 dB in a distance of 1 meter around the enclosure without nanocomposite and with nanocomposite D are shown in Table 2.

Table 2.

The average SPL in sound ranges of 75-80, 80-85, 85-90, 90-95 dB in a distance of 1 meter around the enclosure without nanocomposite and with nanocomposite type D.

Place of measurement	Sound range (dB)	Average SPL (dB)
Enclosure without nanocomposite	80-75	49.83
	85-80	58.29
	90-85	56.43
	95-90	58.86
Enclosure containing nanocomposite	80-75	48.18
	85-80	48.56
	90-85	51.93
	95-90	51.84

According to Table 2, the average SPL in a sound range of 75-80 dB in a distance of 1 meter around the enclosure without nanocomposite and with nanocomposite was obtained as 49.83 and 48.18 dB, respectively; in addition, the noise reduction was about 1.65 dB. The average SPL in a sound range of 80-85 dB in a distance of 1 meter around the enclosure without nanocomposite and with nanocomposite was obtained as 58.29 and 48.56 dB, respectively, and the noise reduction was about 9.73 dB. The average SPL in a sound range of 85-90 dB in a distance of 1 meter around the enclosure without nanocomposite and with nanocomposite was obtained as 56.43 and 51.93 dB, respectively, and the noise reduction was about 4.6 dB. The average SPL in a sound range of 90-95 dB in a distance of 1 meter around the enclosure without nanocomposite and with nanocomposite was obtained as 58.86 and 51.84 dB, respectively, and the noise reduction was about 7.02 dB.

As can be seen in Figure 3, the highest SAC at medium and low frequencies was related to nanocomposites D. Based on the purpose of this study, the results showed that by adding polyester layer, Pan nanofibers and nanoclay separately to SA, the SAC can not be significantly improved at medium and low frequencies. However, by adding organic (nonwoven polyester layer and pan nanofibers) and mineral (nanoclay particles) materials to the SA simultaneously, the SAC was increased especially at medium and low frequencies, and resulted in the improved sound absorption properties.

The reason for such behavior is probably due to the fact that the aerogels are porous with low density and when the aerogels are dispersed in an organic or mineral materials, a composite with high porosity is formed. In addition, when the sound waves hit the specimens, they penetrate through many of tiny pores or holes. As the porosity of composite increases, the complexity of the routes, through which the sound waves pass are increases. Therefore, the pressures caused by air friction in the pores of the silica aerogels results in an increased sound absorption. While the sound waves pass through the pores, the air in the pores begins to vibrate and causes the pores wall to vibrate which consequently attenuates the wave strength. The smaller the pores within the structure, the more energy is absorbed by vibration. The low density of aerogel composites slows the propagation of sound waves within the material, thereby sound absorption is increased.¹¹

Based on Figure 4, the TL of all nanocomposites showed that the TL of nanocomposite D was higher at all frequencies compared to other nanocomposites. The TL of nanocomposite D at high frequencies was higher than at the low frequencies. At low frequency, the highest TL was related to nanocomposite D. The TL value of all nanocomposites reached their peak at a frequency of 6300 HZ. As can be seen in Figure 4, the TL value of all nanocomposites did not increase significantly. Since the TL is mostly influenced by rigidity and high density this phenomenon was predictable.

According to Table 2, the average SPL reduction in the sound range of 75-80, 80-85, 85-90, 90-95 dB in a distance of 1 meter around the enclosure without nanocomposite and with nanocomposite was 1.65, 9.73, 4.5, and 7.02 dB, respectively. On the other hand, the results showed that the highest average SPL reduction was in the sound range of 80-85 dB in a distance of 1 meter. Results of sound measurements showed that using optimum nanocomposite in different sound ranges could significantly reduce the noise.

Conclusion

In this study, the SAC of four nanocomposites were measured and evaluated by impedance tube model BSWA-SWW 422, SW477. The results showed that the SAC of all nanocomposites was increased with increasing the frequency. According to the SAC curve, the SAC of all nanocomposites were higher than 0.6 at high frequencies. The results showed that adding organic and mineral materials simultaneously to the SA could increase the SAC, especially at medium and low frequencies. The results of the TL measurements showed that the TL of the nanocomposite D was higher at all frequencies compared to other nanocomposites. The TL of nanocomposite D at high frequencies was higher than at the low frequencies. At low frequency, the highest amount of the TL was related to nanocomposite D. The results showed that adding organic and mineral materials simultaneously to SA could improve the TL too. The results of average SPL measurements in a distance of 1 meter around the enclosure without nanocomposite and with nanocomposite showed that using nanocomposite D can reduce the noise. Therefore, based on this research, SA/polyester nonwoven layer/pan nanofibers/nanoclay nanocomposite (i.e. type D) had good sound absorption properties, which can be used in different environments. On the other hand, these innovative composite materials can introduce a new branch of material that has good sound properties.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Parvin Nassiri

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Synthesis of a multicomponent silica aerogel-containing nanocomposite for efficient sound absorption properties

Abstract

Keywords

Introduction

Experimental

Materials

Preparation of nanocomposites

Measurement of SAC

Electrospinning apparatus

Measurement of sound pressure level (SPL)

Characterization

Results and discussion

Characterization

Sound absorption coefficient (SAC) of the nanocomposites

Sound transmission loss (TL) of the nanocomposites

Sound pressure level (SPL) of the nanocomposites

Conclusion

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References